Business Segments (Details) $ in Millions		3 Months Ended		6 Months Ended				12 Months Ended
		Jun. 30, 2020 USD ($) a	Jun. 30, 2019 USD ($)	Jun. 30, 2020 USD ($) a Segments		Jun. 30, 2019 USD ($)		Dec. 31, 2019 USD ($)
Business Segments [Line Items]
Number of Reportable Segments | Segments				3
Operating Revenue		$ 243.2	$ 290.4	$ 554.8		$ 647.6
Net Income (Loss) Attributable to ALLETE		20.1	34.2	86.4		104.7
Gain on Sale of U.S. Water Services, After-tax								$ 13.2
Assets		5,760.4		$ 5,760.4				5,482.8
Regulated Operations [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments				3
Operating Revenue		200.8	249.8	$ 466.1		532.0
Net Income (Loss) Attributable to ALLETE		11.1	30.3	68.6		81.8
Assets		4,139.8		4,139.8				4,130.8
Regulated Operations [Member] | Residential [Member]
Business Segments [Line Items]
Operating Revenue		27.4	29.2	68.3		74.4
Regulated Operations [Member] | Commercial [Member]
Business Segments [Line Items]
Operating Revenue		28.8	34.9	66.1		73.8
Regulated Operations [Member] | Municipal [Member]
Business Segments [Line Items]
Operating Revenue		9.0	12.2	19.3		27.6
Regulated Operations [Member] | Industrial [Member]
Business Segments [Line Items]
Operating Revenue		88.0	120.3	206.8		241.9
Regulated Operations [Member] | Other Power Suppliers [Member]
Business Segments [Line Items]
Operating Revenue		27.4	35.2	65.7		74.6
Regulated Operations [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		20.2	18.0	39.9		39.7
ALLETE Clean Energy [Member]
Business Segments [Line Items]
Operating Revenue		17.6	15.4	37.7		32.9
Net Income (Loss) Attributable to ALLETE		4.0	1.9	15.7		7.7
Assets		1,250.2		1,250.2				1,001.5
ALLETE Clean Energy [Member] | Long-term PSA [Member]
Business Segments [Line Items]
Operating Revenue		14.8	12.5	32.1		27.1
ALLETE Clean Energy [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue, Other Than Customer Revenue		2.8	2.9	5.6		5.8
U.S. Water Services [Member]
Business Segments [Line Items]
Operating Revenue		0.0	0.0	0.0	[1]	33.4	[1]
Net Income (Loss) Attributable to ALLETE	[1]	0.0	0.0	0.0		(1.1)
U.S. Water Services [Member] | Point-in-Time [Member]
Business Segments [Line Items]
Operating Revenue	[1]	0.0	0.0	0.0		19.0
U.S. Water Services [Member] | Contract [Member]
Business Segments [Line Items]
Operating Revenue		0.0	0.0	0.0	[1]	9.2	[1]
U.S. Water Services [Member] | Capital Project [Member]
Business Segments [Line Items]
Operating Revenue		$ 0.0	0.0	$ 0.0	[1]	5.2	[1]
Corporate and Other [Member]
Business Segments [Line Items]
Number of Operating Segments | Segments				2
Land in Minnesota (Acres) | a		4,000		4,000
Operating Revenue		$ 24.8	25.2	$ 51.0		49.3
Net Income (Loss) Attributable to ALLETE	[1]	5.0	2.0	2.1		16.3
Gain on Sale of U.S. Water Services, After-tax						11.1
Assets		370.4		370.4				$ 350.5
Corporate and Other [Member] | Long-term Contract [Member]
Business Segments [Line Items]
Operating Revenue		22.5	21.2	44.9		41.4
Corporate and Other [Member] | Other [Member]
Business Segments [Line Items]
Operating Revenue		$ 2.3	$ 4.0	$ 6.1		$ 7.9

[1] In March 2019, ALLETE sold U.S. Water Services. The Company recognized a gain on the sale of $11.1 million after-tax during the six months ended June 30, 2019, which is reflected in Corporate and Other.